Showing papers on "Mullite published in 1983"

Infrared-transparent mullite ceramic

Abstract: Mullite ceramic, transparent in the infrared, was prepared by hot-pressing and hot-isostatically pressing starting materials derived from alkyloxides. A composition with 72.3 wt% Al2O3 yielded transparent, submicrometer grain size bodies at 1630°C, whereas higher temperatures produced glass-containing microstructures. A composition with 76 wt% A12O3 formed precipitates of α-Al2O3 at the consolidation temperature, which could be removed by subsequent annealing between 1800° and 1850°C. Spectral transmittance and absorption coefficients of the bodies are reported. The formation of the second phases was linked to phase equilibria and grain growth that promoted compositional equilibration of the mullite phase. The results suggest adjustments to phase boundaries in the high-temperature segment of the SiO2-Al2O3 phase diagram.

Mechanical Properties of Mullite

Abstract: The mechanical properties of alkoxy-derived, high-purity, translucent, theoretically dense mullite (3AI2O3.2SiO2) were investigated over the temperature range room temperature to 1500°C. Large agglomerates were found to contribute to the formation of porosity nests which act as strength-controlling flaws at room temperature as well as at high temperatures. Despite the slow crack growth above 1300°C, a slight increase in fracture stress and a large increase in KIc were observed up to 1500°C. These increases are explained by the dominance of energy dissipation through plastic relaxation in the plastic zone over grain-boundary sliding due to the presence of the glassy phase.

Aluminum titanate-mullite ceramic articles

Abstract: This invention is directed to the production of sintered ceramic articles wherein aluminum titanate and mullite constitute the predominant crystal phases and wherein the microstructure thereof evidences grain boundary and intracrystalline microcracking. The articles have base compositions encompassed within the area I, J, K, L, M, I of the drawing to which 0.5-5% Fe 2 O 3 and/or 0.5-5% rare earth metal oxide may be added.

Solid Solution Range and Microstructures of Melt‐Grown Mullite

Abstract: The solid solution range of melt-grown mullite was examined by crystal-chemical methods. The maximum Al/sub 2/O/sub 3/ content as determined by EDX was approx. =83.6 wt %, 75 mol %, or the nominal composition 3Al/sub 2/O/sub 3/ X SiO/sub 2/. For samples of overall composition 81 to 83 wt% Al/sub 2/O/sub 3/, extra lines indicating crystallographic superstructure appeared in Guinier X-ray patterns. The corresponding TEM microstructure consisted of a mullite matrix finely twinned on (001), the twins being 0.02 to 0.10 ..mu..m wide, with oriented exsolutin of ..cap alpha..-Al/sub 2/O/sub 3/, often twinned, also being present. The analogy between mullite superstructure and that of plagioclase feldspars, as well as the relevance of these findings to the SiO/sub 2/-Al/sub 2/O/sub 3/ metastable phase equilibria are discussed.

Investigation of superstructures in mullite by high resolution electron microscopy and electron diffraction

Abstract: Superstructures in synthetic mullite, Al4+2xSi2−2xO10−x, prepared using the zone melting technique, are studied by high resolution electron microscopy (HREM), electron diffraction, and in situ energy dispersive X-ray spectroscopy. For x=0.40 composition HREM images indicate only short-range order. Near x=0.5 composition mullite has an antiphase domain structure with antiphase boundaries (APB's) oriented parallel to (100). For higher Al-content the APB's run in non-rational orientations which are slightly inclined against the {601}-planes. We propose models for the superstructures in which oxygen vacancies are arranged in channels parallel to the crystal b-axis. The models are supported by a one-to-one match between observed and computer-simulated images. An Al-rich limit of x=2/3 for mullite is deduced on the basis of stereochemical considerations. Different ordering schemes for the tetrahedral cations are believed to result in the orientation change of the APB's near x=0.5 composition.

Microstructure of Sintered Mullite-Zirconia Composites

Abstract: Fused mullite, pure and with 10 to 25 vol% ZrO2 added, was milled in an attritor and sintered in air at temperatures near 1600°C to a dense fine-grained ceramic The ZrO2 promoted densification and retarded grain growth of the mullite phase Transmission electron microscopy showed that compositions containing ZrO2 were substantially free of glass, whereas pure mullite bodies contained a glassy phase The ZrO2 formed inter granular, isolated particles

Twin Boundaries in Monoclinic ZrO2 Particles Confined in a Mullite Matrix

Abstract: Internal defects and twin structures exist in thermally transformed monoclinic (m)-ZrO2 particles confined in a mullite matrix. The defects were analyzed by transmission electron microscopy. Twin boundaries on (100), (110), (001), and (011) were found. Frequently, (100) and (110) twins formed complicated mosaiclike structures. Some particles could be divided into two or more twinned sections with well-defined mutual orientation relations. Domains of closure were observed at the particle-matrix interface. The observations are discussed in terms of a simple model which considered (i) the anisotropic volume increase of ZrO2 during the t→m transformation, (ii) the anisotropic thermal expansion coefficient of ZrO2, and (iii) the rather isotropic thermal expansion coefficient of the mullite matrix.

Cadmium telluride films on foreign substrates

Abstract: Thin films of cadmium telluride have been deposited on mullite and tungsten‐coated graphite substrates at 500–700 °C by the direct combination of cadmium and tellurium in a hydrogen atmosphere Their microstructure and crystallographic properties were studied The importance of controlling the Cd/Te molar ratio in the reaction mixture to obtain nearly stoichiometric films was demonstrated The electrical properties of nonstoichiometric and nearly stoichiometric films on mullite substrates were measured by the van der Pauw technique Schottky barriers were used to measure the electrical properties of cadmium telluride films on W/graphite substrates The effective intragrain minority carrier diffusion length in n‐type films was measured by the scanned electron beam method using a Schottky barrier structure

Refractory sliding nozzle plate

Abstract: A refractory for use in casting of molten metal which essentially comprises refractory material comprising corundum and monoclinic zirconia as principal mineral phase and including Al 2 O 3 and ZrO 2 , silicon-containing powder and carbon-containing powder and which optionally includes first additional refractory material comprising mullite, baddellite and corundum as principal mineral phase and including Al 2 O 3 , ZrO 2 and SiO 2 , one or more second additional refractory materials and/or alumina.

Linear Thermal Expansion Coefficients of Mullite‐Matrix Aluminosilicate Refractory Bodies

Abstract: Twenty-seven mullite-matrix aluminosilicate refractory bodies in three different refractory systems were synthesized and their thermal expansions were measured and then correlated with their corundum and mullite contents by regression analyses. Thermal expansion coefficients were also calculated from the physical properties of the individual constituent phases. It is concluded that the best estimator of the thermal expansion of these materials is the regression analysis based on the volume percentage mullite content and that those bodies based on tabular alumina aggregates are the most predictable. For the best predictions, only a single raw material system, or very similar raw material systems can be considered.

Structural Characterization of the Spinel Phase in the Kaolin‐Mullite Reaction Series Through Lattice Energy

Abstract: Kaolinite undergoes structural transformation on heating. X-ray photographs reveal the existence of a spinel-type phase when kaolin is heated at 980°C. The kaolinite decomposes into a spinel phase with the expulsion of silica. A controversy arises as to whether the spinel phase is γ-Al2O3 or Si-Al spinel. Calculating the lattice energies of the structures confirms that the spinel phase is γ-Al2O3 and not Si-Al spinel, as proposed earlier. The heat involvement in phase transformation, as obtained from experimental observation at 980°C, is also explained in the light of lattice energies.

Thermal decomposition of kyanite single crystals

Abstract: Single crystals of kyanite (Otavi-Mine, South Africa) were heated gradually up to 1500°C and investigated at room temperature by means of optical microscopy, SEM and X-ray methods. The aim of the present paper is to look for preferred orientations of precipitated mullite that possibly occur during the thermal treatment. This may give insights in the decomposition mechanism. The decomposition of kyanite proved to be strongly dependent on the furnace atmosphere. In alkali-free atmosphere mullite and vitreous silica develop with a distinct texture of mullite with six orientations that can be explained by common [A1O6]octahedra of kyanite and mullite. Lattice constants measurements suggest the composition 3 Al2O3·2 SiO2 (3∶2-mullite) for the precipitated mullite. In the presence of Na-ions corundum appears together with a series of transition phases similar to δ-,γ- and ϑ-Al2O3 with rather good orientations. Occasionally oriented cristobalite occurs. All these orientations are interpreted as due to the pseudo-cubic oxygen packing of the kyanite structure. At 1500°C all transition phases disappear and only corundum and secondary formed mullite with preferred orientations are present.

Hydrogen storage material

Abstract: PURPOSE:To obtain a hydrogen storage material having superior occluding and releasing properties for gaseous hydrogen by sticking a thin film of an alloy of a rare earth metal with Ni on a substrate. CONSTITUTION:Thin films of <=10mu, more preferably 0.1-8mu of an alloy of a rare earth metals with Ni are stuck on a substrate of quartz, alumina, mullite, glass, a high polymer film (for example, PET, polyimide) or the like by a vacuum deposition, sputtering or ion beam depositing method. The rare earth metal- Ni alloy is an alloy of La, Ce, Sm, etc. and Ni, and may contain about 1- 50atom% other metals, for example, Mo, Co, Fe, Cr, Cu, etc. More specifically, such metals are LaNi5, MM(Ni0.8Co0.2)2 (MM is Mischmetal), Ce (Ni0.7Mn0.3)4.5, Sm(Ni0.3Fe0.3)5, etc.

Influence of TiO2, Fe2O3 and Na2O on the structural changes in fused mullite

Abstract: On etudie la solubilite de TiO 2 , Fe 2 O 3 et Na 2 O dans la mullite fondue, traitee a 1650°C, on determine les facteurs structuraux de la mullite, le mecanisme de formation des solutions solides et les modifications dans la porosite et la densite des echantillons

Structural and microstructural transitions during phase separation and crystallization of Al2O3-SiO2-P2O5-Y2O3 glass

Abstract: A highly refractory glass in the system Al2O3-SiO2-P2O5-Y2O3 has been designed and produced such that, upon heating, an essentially fully crystalline glass—ceramic evolves containing mullite (nominally 3Al2O3·2SiO2) and xenotime (YPO4) as the final principal phases. Phase separation in this glass occurred during cooling from the melt and continued during annealing. XPS of the Al 2p, P 2p and the Y 3d electrons revealed that the average chemical environment of each of these elements is measurably different in the annealed glass and in the completely crystallized material. This indicates that the compositions of the separated glass phases are very different from those of the crystal phases which form from them. Additional rearrangement of the glass structure was observed at 1173 K. Extensive formation of mullite was initially detected at 1223 K and was followed by the crystallization of xenotime and the transient compounds of Y4Al2O9, Y2Si2O7, AlPO4 and YP5O14. The optimum crystal nucleation and crystallization temperatures of 1173 and 1473 K, respectively, were determined from DTA, XRD, SEM and TEM studies.

The Significance Of The Mullite Phase In A Silver Catalyst For The Oxidation Of Ethylene Into Ethylene Oxide

Abstract: An α -alumina support with suitable pore structure and good heat transfer properties has been prepared. Silver dispersed in uniform grains of suitable size is important for the selectivity of the silver catalyst. We tried various preparative procedures to disperse silver on the support, and found out that the master key to success was the addition of a mullite phase to the α-alumina support. On recognizing the significant role played by the mullite phase, we prepared the support for the silver catalyst from corundum and alumina-silica gel. Satisfactory results have been obtained.

Transparent glass-ceramics containing mullite and production thereof

Abstract: A substantially transparent glass-ceramic characterised in that it contains mullite as the predominant crystal phase, which may be prepared from a precursor glass capable of being melted at temperatures no higher than 1650°C and which may be crystallized in situ at high viscosities of from 109 to 10" poises (from 108 to 10" Pa.s), and in that: it comprises, expressed in terms of weight percent on the oxide basis, from 10 to 70% SiO2, from 7 to 40 % B2O3, from 14 to 50% Al2O3, from 0 to 35% RO, wherein RO comprises at least one of the following MgO, CaO, SrO, BaO and PbO and from 0 to 30% R20, wherein R20 comprises at least one of the following Li20, Na2O, K2O, Rb20 and Cs20, the mole ratio of Al2O3 : RO+R20 being greater than 1.3:1 is disclosed. A process for the production of such a glass ceramic which comprises: (a) melting an appropriate batch for a precursor glass; (b) simultaneously cooling the melt to a temperature at least below the transformation range thereof and shaping the glass as desired; (c) exposing the glass to a temperature of from 700 to 950°C to cause the generation of crystallization In situ; and (d) cooling the crystallized glass to room temperature is also disclosed. Where 0.01-1% Cr203 is included in the composition, the glass-ceramic will exhibit broad absorption over the visible region of the radiation spectrum, strong fluorescence in the red and near infrared portions of the spectrum where exposed to ultraviolet and/or visible radiation, and relatively little overlap of the absorption and fluorescent spectra. Thus, the present invention provides advantages over the prior art.

Preparation of gas sensitive element

Abstract: PURPOSE:To restrain a change in the lapse of time and also, to improve remarkably sensitivity, by forming a catalyst layer containing a catalyst and a carrier on a layer of a gas-sensitive material consisting of an oxide semiconductor which is formed on the surface of an insulating substrate with a pair of electrodes. CONSTITUTION:A mullite compound which is made by mixing SiO2 and Al2O3 at 2:3 weight ratio and calcining it temporarily is used for a carrier of a catalyst layer. Moreover, the mullite compound is crushed and is mixed with a solution of H2PtCl6.6H2O etc. Said mixture is dried and is crushed and is calcined. The catalyst is obtained by crushing said calcined mixture. The mullite compound functions both actions as the auxiliary catalyst and the carrier because SiO2-Al2O3 itself is an acidic catalyst and has heat resistance. A ZnO oxide semiconductor is coated to an insulating substrate 1 provided with a pair of electrodes 2 as a gas sensitive material 3 and the gas sensitive material layer is formed by calcining said substrate after drying. The catalyst is slurried by using an adequate solvent and is coated to the surface of the gas-sensitive material 3 and is dried. The gas sensitive element is obtained by forming a catalyst layer 4 after calcining said gas-sensitive material 3.

Thermomechanical properties of mullite-corundum refractories based on fused mullite

Abstract: A study was made of the strength and deformation behavior of mullite-corundum refractories with various concentrations of fused mullite. Analysis of the range of properties, taking into account the demands placed on refractories for service in different components of the structure of high temperature blast furnace stoves, suggests that articles based on compositions Nos. 2–4 may be used in the lower parts of the combustion chambers, since they possess outstanding deformation properties; and articles based on compositions Nos. 1, 2, and 4 may be used in the hot blast connecting pipes and gas burners, where it is also necessary to provide a high thermal-shock resistance in the refractory.

Dry friction material having high friction coefficient

Abstract: PURPOSE:To obtain the dry friction material which stably exhibits a high friction coefficient from a low-temperature zone to a high-temperature zone, by using a sintered body of specified composition prepared by adding carbon, sulfide, glass, silica particles etc. to a copper-based matrix alloy. CONSTITUTION:The dry friction matreial having a high friction coefficient and comprising, by wt%, 70-95% a copper-based matrix alloy containing 5-20% tin or tin and zinc, 3-15% carbon, 0.5-5% one or more of sulfide solid lubricants as molybdenum disulfide and tungsten disulfide, 0.5-10% amorphous silicon oxide (glass) an 1-10% one or more of hard particles having Mohs' hardness abov above 4 such as silica, alumina and mullite is obtained by mixing desired amoun ed amounts of said powdery components, press-forming the powdery composition and hot press-sintering it. Among said components, the carbon content, the sulfide and the glass content inhibit hot-melt adhesion in a low-temperature zone, in a medium-temperature zone and in a high-temperature zone, respectively, and these components concurrently exhibit the effect to stabilize a friction coefficient.

Manufacture of needlelike mullite crystal powder

Abstract: PURPOSE:To manufacture needlelike mullite crystal powder by hydrothermally treating a substance contg. Al and a substance contg. Si at high temp. and pressure using water as a medium and carrying out calcination. CONSTITUTION:A substance contg. Al is mixed with a substance contg. Si so as to make the atomic ratio of Al/Si >=about 2, and hot water regulated to >=550 deg.Chigh temp. and >=10kg/cm high pressure is allowed to act on the mixture. The resulting powdered precipitate is taken out by filtration, dried, and calcined to manufacture needlelike mullite crystal powder.